1. Field of the Invention
The present invention relates to a semiconductor laser module in which a wavelength of laser light emitted from a semiconductor laser element is selected and locked by using an optical wavelength selection element, and the laser light having the selected wavelength is converted to wavelength-converted laser light such as a second harmonic by using an optical-waveguide type wavelength conversion element.
2. Description of the Related Art
Optical wavelength conversion elements which comprise a substrate made of a ferroelectric crystal exhibiting a nonlinear optical effect, an optical waveguide formed on the substrate so as to extend in a certain direction, and domain-inverted portions periodically formed along the optical waveguide are known, for example, as disclosed in Japanese Unexamined Patent Publication (JUPP) No. 10(1998)-254001, which is assigned to the present assignee. Such optical wavelength conversion elements convert a fundamental wave propagating in the optical waveguide, to a second harmonic or the like. In the domain-inverted portions, the direction of spontaneous polarization of the substrate is inverted.
JUPP No. 10(1998)-254001 also discloses a semiconductor laser module (or an optical wavelength conversion module) in which an optical wavelength conversion element is coupled to a semiconductor laser element which emits a laser beam as a fundamental wave. JUPP No. 10(1998)-254001 further discloses a semiconductor laser module in which an external resonator including an optical wavelength selection element (such as a narrow-band-pass filter) is combined with a semiconductor laser element which emits a laser beam as a fundamental wave so that an oscillation wavelength of the semiconductor laser element is locked at a desired wavelength.
Japanese patent application No. 11(1999)-345724, which is also assigned to the present assignee, discloses a semiconductor laser module in which a semiconductor laser element emitting a laser beam as a fundamental wave, an optical wavelength selection element selecting a wavelength of the laser beam, and an optical wavelength conversion element converting the wavelength of the laser beam are directly coupled. The contents of Japanese patent application No. 11(1999)-345724 are incorporated in this specification by reference.
In the above semiconductor laser module, an optical-waveguide-type optical wavelength selection element is coupled to a semiconductor laser element directly or through an optical wavelength conversion element. Therefore, light having a phase-matching wavelength in a wavelength conversion portion in the optical wavelength conversion element can be selected by using the optical-waveguide-type optical wavelength selection element, and fed back to the semiconductor laser element. Thus, the oscillation wavelength of the semiconductor laser element can be selected and locked, and stable wavelength-converted light can be obtained.
In semiconductor laser modules which comprise an optical wavelength conversion element including a periodically formed domain-inverted portions (periodic domain-inverted structure), the oscillation wavelength of a semiconductor laser element as a light source of a fundamental wave is required to be precisely locked at a predetermined wavelength at which phase matching of wavelength-converted light occurs in the optical wavelength conversion element. In addition, demands for precisely setting the wavelength of wavelength-converted light at a desired value exist in a wide range of applications in which semiconductor laser modules including optical wavelength conversion elements of other types are used. In order to precisely lock the wavelength of wavelength-converted light, it is necessary that the oscillation wavelength of the semiconductor laser element as a light source of a fundamental wave is precisely locked at a predetermined wavelength.
The range of wavelengths which each semiconductor laser element can emit is determined by the peak width of the gain spectrum of the semiconductor laser element. Therefore, when the semiconductor laser element is manufactured, the (chemical) composition and the thickness of the active layer of the semiconductor laser element must be precisely controlled so that the wavelengths which the semiconductor laser element can emit are adjusted in the vicinity of a desired wavelength.
In addition, since, in practice, efficiencies in wavelength conversion and optical coupling, transmittances of filters, and the like are different in each semiconductor laser module, reflectances in external wavelength locking systems, when viewed from semiconductor laser elements, vary with the individual semiconductor laser modules. Further, the gain spectra of oscillations in the semiconductor laser elements vary with the reflectances. Generally, when the variation in the reflectance increases, the wavelength locking becomes difficult. However, it is desirable that the oscillation wavelength of each semiconductor laser element can be locked in a wide wavelength range, i.e., the oscillation wavelength can be adjusted in the wide wavelength range. In addition, when the wavelength range in which the oscillation wavelength of each semiconductor laser element can be locked is wider, conditions concerning the composition and the thickness of the active layer of the semiconductor laser element can be more eased, and the semiconductor laser element for use in a semiconductor laser module can be manufactured at a higher yield rate.
Nevertheless, the wavelength ranges in which the oscillation wavelengths of the semiconductor laser elements used as light sources of fundamental waves in the conventional semiconductor laser modules can be adjusted are not sufficiently wide. Therefore, in the conventional semiconductor laser modules using a semiconductor laser element as a light sources of a fundamental wave, the adjustment operations required for the wavelength locking are complicated, and the control of the composition and thickness of the active layer of the semiconductor laser element is difficult.